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J. Cell Biol.,
Volume 143, Number 6, December 14, 1998 1673-1690
Institute of Biochemistry, University of Zurich, CH-8057 Zurich, Switzerland
Neural cell adhesion molecules composed of
immunoglobulin and fibronectin type III-like domains
have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic
interactions across the extracellular space (axonin-
1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1-NgCAM) that occurs exclusively
in the plane of the same membrane (cis-interaction).
Using domain deletion mutants we localized the
NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized
in the Ig domains 2-4 and the third FnIII domain. The
NgCAM-NgCAM interaction could be established simultaneously with the axonin-1-NgCAM interaction.
In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at
cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in
which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of
NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.
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